1. Field of the Invention
The present invention relates to a scanning optical apparatus, and particularly relates to a scanning optical apparatus that can be employed in an electrophotographic-type image forming apparatus.
2. Description of the Related Art
Electrophotographic-type image forming apparatuses form electrostatic latent images upon a photosensitive drum by scanning the drum with a light beam using an exposure apparatus. It is generally desirable for the diameter of the light beam spot formed upon the photosensitive drum to be constant regardless of what position upon the photosensitive drum the light beam spot is in. This is because if the diameter of the light beam spot is not constant, the thickness of lines will decrease or increase depending on the scanning position in the main scanning direction on the photosensitive drum. Accordingly, the exposure apparatus is provided with an optical system for collecting light beams output by the light source (for example, an f-θ lens or the like).
However, even if the light is collected using an f-θ lens, a slight amount of focus error will occur depending on the scanning position. For example, the spot diameter has differed by several μm between the scanning start side, the central area, and the scanning end side on the photosensitive drum.
If the exposure apparatus has a resolution of 600 dpi (that is, a spot diameter of 42 μm), optical adjustments are made so that the desired spot diameter is obtained in the central area of the scanning position, resulting in a spot diameter on the scanning start and end sides of approximately 46 μm. If the resolution is low, namely, up to approximately 600 dpi, this difference of 4 μm in the spot diameter can be ignored in terms of the problems it presents to image quality.
However, at high resolutions such as 2400 dpi (with a desired spot diameter of 10.5 μm), a 4 μm expansion in the spot diameter can no longer be ignored. The reason for this is that while 4 μm only causes the spot diameter to expand by 10% at 600 dpi, the same 4 μm causes the spot diameter to expand by approximately 40% at 2400 dpi. Furthermore, because the integrated amount of light when forming a single spot is constant, a spot whose diameter has expanded by 40% also contains 40% less light. Therefore, the light amount on the scanning start and end sides decreases relative to the light amount in the central area of the scanning position, which leads to a decrease in the darkness of the image. Thus expansion of the spot diameter caused by focus error in the optical system leads to a drop in image quality at high resolutions.
A technique in which a collimator lens, a cylindrical lens, or the like is moved in the optical axis direction (Japanese Patent Laid-Open No. H08-334710, or “Patent Document 1” hereinafter), a technique in which a light source, optical element, or the like is moved in the optical axis direction (Japanese Patent Laid-Open No. 2001-091882, or “Patent Document 2” hereinafter), and so on have been proposed as techniques for focusing the spot.
However, the methods disclosed in Patent Documents 1 and 2 are both nothing more than methods for performing focus optically. In other words, in order to focus the spot on any one scanning position, it is necessary to move the light source, optical element, lens, or the like at a high speed in accordance with the scanning position of the light beam upon the photosensitive drum. Because a scan occurs in an extremely small amount of time, it is difficult to implement such high-speed driving, and even if such high-speed driving could be implemented, it would incur high costs.